Electromagnetic flowmeters (which are sometimes referred to as magnetic flowmeters or “mag meters”) measure the flow rate of an electrically conductive fluid through a flowtube. In a conventional electromagnetic flowmeter, electrical coils are mounted on opposite sides of the tube and energized to produce an electromagnetic field perpendicular to the direction of fluid flow in the flowtube. When a conductive fluid flows through the electromagnetic field, an electric field is generated in the fluid that can be measured to determine the flow rate. In a typical set up, a pair of electrodes extends through the wall of the flowtube and into the fluid for measuring the strength of the electric field to determine the flow rate. Sometimes additional electrodes extend through the wall of the flowtube into a conduit therein in order to provide empty pipe detection or to ground the liquid. Each point where an electrode extends through the flowtube wall into the conduit requires a so-called process penetration. As illustrated in FIG. 1, a conventional electrode 15 includes a head 19 and a shank 21 extending away from the head. The shank 21 is inserted into an opening 17 forming the process penetration so the head 19 is in the conduit 7 formed by the flowtube 3 and so the shank extends through the flowtube wall 5. A fastener 25 (e.g., a threaded nut) is used to hold the electrode is in this position.
The process penetrations should be sealed to keep the fluid from leaking into the process penetration as it flows through the flowmeter 1. One way this is done is to provide serrations 23 on the back side of the head 19 of each electrode 15. The inner surface of the flow tube 3 is commonly lined with an electrically insulating and chemically resistant liner 11 to prevent the conductive fluid from creating a short circuit between the electrode 15 and the flowtube wall 5, which is commonly made of an electrically conductive material such as metal. Thus, when the nut or other fastener 25 is tightened, the serrations 23 on the back of the electrode head 19 dig into the liner 11 and form a seal between the head of the electrode 15 and the liner. This seal is known as the primary seal. The shank 21 of the electrode 15 is insulated from the electrically conductive part of the flowtube wall 5 by an insulating sheath 31 surrounding at least the segment of the shank that is adjacent the conductive flowtube wall. If fluid leaks past the primary seal, it will also have to flow past the insulating sheath 31 to completely escape through the process penetration. As a result, fluid can leak through the liner 11 and contact the flowtube 3 without any evidence of the leak being visible from outside the flowtube 3.
The fluids metered by electromagnetic flowmeters can include very corrosive and/or caustic materials. In some processes the fluids can also be at a fairly high temperature when they flow through the electromagnetic meter, which can increase the rate at which the fluid causes damage to other materials (e.g., the flowtube wall 5). The present inventors have noted that fluids may leak past the primary seal and cause extensive corrosion of the flowtube wall 5 before a leak is detected. This can present a significant hazard because damage to the flowtube wall 5 can impair the pressure containment capability of the flowtube. Thus, the leak may not be detected until the flowtube bursts and releases the corrosive fluid in a catastrophic failure.